1. Field of the Invention
This invention relates to rotary power transmission systems, and more particularly, to power transmission systems utilizing a plurality of clutches for varying the gear ratio of a transmission by selectively engaging one clutch while substantially simultaneously disengaging another clutch, and even more particularly, this invention relates to a control system for such clutches that prevents simultaneous engagement of both clutches in the event that one of the clutches fails to disengage.
2. Description of the Prior Art
Transmissions that shift gears by engaging one clutch while simultaneously disengaging another clutch are known. However, many of such transmissions, particularly those using multiple plate clutches, have the disadvantage of rough shifts that jolt the vehicle during the shift, particularly if the shift is made under load, such as, for example, in a heavily loaded tractor. In order to avoid such jolt, it has been proposed for the two clutches to provide thereto a certain amount of clutch overlap whereby one or both clutches are forced slightly to slip for an instant, and wherein the clutch that carries the load prior to the shift is not disengaged until the newly engaged clutch has engaged sufficiently to transmit a substantial amount of torque.
Although such a clutch overlap control system causing temporary clutch slippage does significantly smooth out the shifting characteristics of the vehicle, there is a possibility that, in the event of a failure of a component of the control system, one of the clutches could fail to disengage after a shift. This would result in a situation wherein one or both of the engaged clutches could continue to slip with severe damage to one or more of the clutches, or to other components of the engine or transmission.